1. Document Disclosure Reference
The application for patent is based on a disclosure filed on Jun. 26, 1995, as Applicant's previously filed Disclosure Document No. 387,572, under the Document Disclosure Program.
2. Field of the Invention
The invention relates to the shielding of fluorescent ballasts by using a fluorescent lamp ballast case made of a ferromagnetic material to shield it from electromagnetic fields, particularly the magnetic component of the electromagnetic fields up to frequencies of about 100 Kilohertz. Additionally, the fluorescent lamp ballast case may be made of steel or aluminum and lined on the inside or outside with thin ferromagnetic foil alloys.
As used herein and in the appended claims, the term “ballast case” refers to a “fluorescent lamp ballast case”. The magnetically shielded fluorescent lamp ballast case can be used for both core coil fluorescent lamp ballasts and for solid state electronic fluorescent lamp ballasts.
3. Description of Related Prior Art
Related prior art concerns itself with the shielding of the fluorescent lamp, or the entire fluorescent fixture, or the reduction of the electromagnetic interference by providing a current path external to an arc discharge lamp so as to produce a magnetic field generally in opposition to the magnetic field generated by the current in the arc discharge, particularly for circular fluorescent lamps. None of the prior art relates to the shielding of the fluorescent ballast from the magnetic component of the electromagnetic field at frequencies of up to about 100 Kilohertz by the use of ferromagnetic shielding materials in the fluorescent lamp ballast case.
For example, Fisher et al. (U.S. Pat. No. 4,684,810, issued Aug. 4, 1987) discloses a cylindrical shield affixed to the opposed marginal terminal end portions of a fluorescent light tube. The shield each include a layer of magnetic substances with intersect X-rays emitted by the cathode of the tube to avoid the harmful effects that are brought about by the X-rays impinging upon people located nearby.
Levin (U.S. Pat. No. 5,587,167, issued Jun. 28, 1971) discloses a novel radio frequency shielding for fluorescent lights comprising a continuous strip of flexible film coated with a layer which is electrically conductive and capable of transmitting light, said film being formed into a closure and surrounding said fluorescent lamp.
Ott (U.S. Pat. No. 3,885,150, issued May 20, 1975), discloses an improved radiation shielded luminaire utilizing gas discharge lamps. Shielding of radio frequency radiation is provided by a grounded superimposed screen and louver assembly. Additional shielding around the cathode area of the lamp shields radiation in the frequency ranges of X-ray and infrared radiation.
Roberts (U.S. Pat. No. 4,409,52l, issued Oct. 11, 1983) teaches that the electromagnetic interference produced by arc discharge lamps and other devices operating at frequencies in excess of 15,000 Hertz can be reduced by providing a current path external to the envelope containing the discharge, the current flow in the path being oriented so as to produce a magnetic field generally in opposition to the magnetic field generated by the current in the arc discharge. This invention is particularly applicable to circular fluorescent lamps with a centrally disposed ballast operating at relatively high frequencies.
Ferromagnetic materials have been used for magnetic shielding of a number of devices. For example, Katz (U.S. Pat. No. 5,336,848, issued Aug. 9, l994) teaches the use of Co-Netic alloy for the shielding of a lap-top computer. Additionally, Crutchfield (U.S. Pat. No. 5.357.061, issued Oct. 18, 1994)) teaches the use of Co-Netic alloy for the shielding of a digitizer tablet. Henry (U.S. Pat. No. 4,625,573, issued Dec. 2, 1986), teaches the use of various ferromagnetic alloys for a magnetically shielded borehole core drilling device. Furthermore, Popovic et al. (U.S. Pat. No. 4,963,827, issued Oct. 15, 1990) teaches the use of mu-metal for an intermittently activated magnetic shield arrangement for reducing noise and offsets in solid state magnetic field sensors.
Petrina (U.S. Pat. No. 4,393.435, issued Jul. 11, 1983) discloses a ballast coil, transformer, and power factor capacitor that are plug mounted on a PC-board which is mounted in a frame being covered to provide a complete, unpotted structure. However, Petrina does not disclose any electromagnetic shielding of the ballast case.
Ozaki et. al. (U.S. Pat. No. 5,607.228) teaches an automtive headlamp capable of effectively shielding the electromagnetic waves generated by the discharge lamp of a light source, using a conductive layer composing several layers of durable magnetic plating. Ozaki shields the lamp, not the ballast. Also, Ozaki uses an electrically conductive shielding material, not a magnetic shielding material. Ozaki uses layers of pure metal plating; not an alloy of nickel and iron.
Blocher et al. (U.S. Pat. No. 5,446,617, issued Aug. 29, 1995) discloses a ballast circuit and grounding structure for electrically grounding a ballast circuit to a housing and for capturing transmitted RFI and EMI therefrom. Good shielding efficiency for plane waves or electric (high impedance) fields is obtained by using materials of high conductivity such as copper and aluminum. However, low-frequency magnetic fields are more difficult to shield because the reflection and absorption losses of non-magnetic materials, such as aluminum, may be insignificant. Consequently, to shield against low-frequency magnetic fields, it is necessary to use magnetic shielding materials.
4. Theory of the Invention
In the design of the electrical circuit for a fluorescent lamp ballast, a transformer, inductor, or other magnetic components are included in the ballast. If a fluorescent lamp ballast contains such components, then alternating current flowing through these components gives rise to electromagnetic fields of various frequencies.
In a core coil fluorescent lamp ballast, the magnetic fields are 60 Hertz. There may be multiples of the 60 cycle magnetic fields produced from harmonics in the circuitry.
In the last ten years, the fluorescent lamp ballast industry has been shifting to solid state ballasts. These fluorescent lamp ballasts contain rectifier and inverter circuitry. The inverter circuit provides alternating current generally between 20,000 Hertz and 50,000 Hertz to drive the fluorescent lamp. There are three types of inverter circuits. The self excited inverter has the input winding, the output winding, and the feedback winding on the same core. The flip flop occurs because of the saturation of the core. These circuits produce strong microphonics, and the external fields are high because of saturation of the main core on each half circuits. The self excited inverter has the input winding, output winding, and feedback winding on the same core. The flip flop occurs because of the saturation of the core. These circuits produce strong microphonics, and the external fields are high because of saturation of the main core on each half cycle. A second type, the separate oscillator excited inverter, has a transformer designed to saturate at 40 Hertz. However, because the transformer is operating at 60 Hertz, it does not saturate at that frequency. It uses a separate oscillator running at 60 Hertz feeding a power transistor. Typically, these circuits produce 20 DB less of external magnetic fields and microphonics. A third type, the self excited with a separate saturable core, has the saturable core wired between the feedback winding of the main core and input to the power transistor. A small transformer in the feedback circuit does the saturating but carries no substantial power.
Many people working under or near fluorescent lighting may feel tired, fatigued, stressed out, or having headaches, eyestrain, or blurred vision. The cause of these problems may be unpolarized illumination or poor quality light sources that have correlated color temperatures below 5,000 degrees Kelvin. However, even in the presence of polarized illumination using full-spectrum lamps, a number of very sensitive individuals may be affected by extremely low levels of electromagnetic fields. Both the 60 Hertz fields and the 20,000 to 50,000 Hertz fields may affect people. It is suspected that field strengths as low as 1 microgauss or lower may affect sensitive individuals. Thus, there is a need for shielding of the fluorescent lamp ballast case of the fluorescent lamp ballast to attenuate the electromagnetic fields, and in particular, the magnetic components of the electromagnetic fields.